The invention relates to capacitive sensors, in particular the invention relates to touch sensitive capacitive sensors for detecting the presence of an object, such as a user's finger, adjacent a panel.
Capacitive sensors have become increasingly common and accepted for use in human interfaces and for machine control. In the field of home appliances it is now quite common to find capacitive touch controls operable through glass or plastic panels.
Touch sensitive control panels generally comprise a capacitive sensor mounted behind a panel. The capacitive sensor comprises a conductive sensor element in contact with the panel and a measurement circuit for determining the capacitance of the sensor element. The configuration of the sensor element defines the sensitive area of the capacitive sensor. The capacitance of the sensor element is modified by the presence of a user's finger positioned adjacent the panel above the sensitive area. Accordingly, a touch is identified by a measured change in this capacitance.
A known type of capacitive sensor employs a conductive area attached to an underside of a panel as the sensor element. The conductive area may be metal foil or a separate printed circuit board containing metal tracks that form the conductive area. The conductive sensor element on the underside of the touch panel is then electrically connected to the measurement circuit by a contact part. Examples of contact parts used in this type of prior art to connect the measurement circuit with the conductive sensor element on the underside of the touch panel are helical springs, conductive rubber pillars and plug-and-socket arrangements. An example of this kind of sensor is disclosed in DE 201 19 700 Al.
Another known type of capacitive sensor, as described by Platt in U.S. Pat. No. 5,917,165, comprises a touch contact switch with an electrically conductive foam, flexible plastic or rubber body held in compression between the underside of a touch panel and an underlying circuit board. The face of the conductive foam in contact with the panel acts as a sensor element while the conductive rubber body electrically connects to the measurement circuit. This arrangements avoids the need for a separate printed circuit board or conductive foil to form the sensor element. Ingraham, in U.S. Pat. No. 5,087,825, describes a sensor similar to that of Platt.